The present invention relates generally to amplifier circuits and more specifically to complementary metal oxide semiconductor (CMOS) insulated gate field effect transistor linear amplifier circuits.
It is known that a complementary field effect transistor (FET) inverter may be used as an analog amplifier when suitably biased. Such amplifiers, however, have not found wide use in analog applications because of the difficulty of biasing a complementary FET inverter to a suitable operating point. The reason for this difficulty is that the input-output transfer function associated with such amplifiers is characterized in having a relatively narrow region where the output signal changes appreciably in response to changes in the input signal. This is a distinct advantage in digital applications where the resulting insensitivity to signals outside the narrow region provides the amplifier (inverter) with exceptionally high noise immunity compared to other logic families. On the other hand, this relatively narrow region of the transfer function requires precise control of applied bias when a complementary FET inverter is used in analog applications as an amplifier, and precision is difficult to achieve due to the relatively unpredictable nature of the transfer function concerned. This results from many factors including primarily manufacturing mismatch between the P and N channel devices. One method of compensating a CMOS inverter for variations is to include variable resistive elements in series with the P channel device and in series with the N channel device which may be controlled inversed to each other to reach a balance. This method is illustrated in U.S. Pat. No. 3,914,702 to Gehweiler. Although Gehweiler improves the accuracy of the operating point of the inverter, it adds resistance to each of the current paths and thus consequently slows the response time of the inverter.
Thus there exists a need for an amplifier circuit using a CMOS inverter as the output stage which has a fixed operating point and improved response time.